1. Field of the Invention
The present invention relates to an image analysis apparatus, radiation imaging apparatus, image analysis method, and storage medium.
2. Description of the Related Art
Conventionally, a technique for visualizing the interior of an object by irradiating the object with a radiation represented by X-rays and imaging radiation components transmitted through the object is used.
Since the radiation generates scattered rays inside the object, the scattered rays are also imaged together with direct rays transmitted through the object. Thus, an instrument called a grid, which removes such scattered rays, is arranged between the object and radiation image-receiving surface to execute imaging. This grid removes scattered radiation components by alternately arranging a radiation shielding material such as lead and a radiation transmission material such as aluminum or carbon to have a predetermined width. Since the grid removes some direct rays passing through the radiation shielding material upon removing the scattered radiation components, it generates a periodic signal (also referred to as a grid line hereinafter) on an image.
As a method of reducing such periodic signal, a method of moving only the grid in a direction perpendicular to a line during irradiation of the radiation, and reducing line components by an integral effect is available. This method can effectively reduce the grid line, but requires mechanical control for moving the grid, resulting in a large apparatus scale, and a disadvantage in terms of cost.
In recent years, a method of reducing a periodic signal caused by the grid by image processing from imaged data has been proposed as Japanese Patent No. 3445258 by the present applicant.
A method of detecting a periodic signal caused by the grid from an image and executing processing only when the periodic signal is detected in terms of improvement of image quality is also available. For example, Japanese Patent Laid-Open No. 8-293020 discloses a method of detecting a periodic signal caused by the grid. With this method, variances in the vertical and horizontal directions are evaluated by an F-test from a plurality of measurement regions, and the presence/absence and direction of a periodic signal of the grid are detected by voting of the evaluation results.
In order to precisely detect a periodic signal of the grid from an image, it is also effective to remove an undetectable region from measurement regions in advance. For example, in Japanese Patent No. 3445258, an estimated average value of an image is calculated by random sampling, and a region less than the estimated average value (that is, an undetectable region with a low S/N) is excluded from measurement regions, thus improving the detection precision.
However, only a rejection condition of Japanese Patent No. 3445258 with reference to the estimated average value of an image is often insufficient as a condition for improving the detection precision. More specifically, when imaging is done under a condition of a small radiation dose or under a condition in which a radiation is partially shielded by a collimator, the estimated average value of an image becomes small as a whole, and a low-S/N region cannot often be appropriately excluded. In addition, high calculation cost is required since all measurement regions not less than the estimated average value are to be evaluated.
Hence, the present invention provides a technique which can improve the detection precision and can reduce calculation cost by limiting measurement regions in which a periodic signal of the grid can be detected more precisely by relative evaluation based on statistic information of measurement regions.